This invention relates to apparatus for detecting a ground fault of the field winding of a synchronous machine.
In a prior art apparatus for detecting a ground fault of the field winding of a synchronous machine, particularly an alternator, upon detection of a fault, the alternator is not tripped immediately but only an alarm display is made. Because, the field circuit of the alternator is not grounded, a one point ground fault would not result in any serious fault, and only when two or more points of the field winding are grounded simultaneously will heavy faults which can burn out the alternator occur. Accordingly, in the case of a single point ground fault the operator of a power station stops the alternator after consulting with a central supervisory station and after performing various preparatory procedures.
However, the increased capacity of an alternator in recent years makes it impossible to disconnect the alternator from the power system on the ground of a simple fault because disconnection of the alternator results in a large disturbance of the power system. However, when the operation of the alternator is continued is spite of the occurrence of a low resistance ground fault of its field winding there is a danger of causing grounding at two or more points.
For this reason, it has been desired to develop a protective system which gives an alarm but permits the operation of the alternator when a high resistance field ground fault occurs which would not damage the alternator seriously, even when the fault develops into a two point grounding, but immediately disconnects the alternator in case of a low resistance field ground fault which may result in a serious fault. However, it has been difficult to judge the magnitude of the grounding resistance during the operation of the alternator.